1. Field of the Invention
The present invention relates to an ultrasonic microscope for observing an internal defect of a sample such as an IC or FRP (Fiber-Reinforced Plastic).
2. Description of the Related Art
A conventional ultrasonic microscope in which only an echo component at a predetermined depth from the surface of a sample can be extracted from an echo reflected from a sample is known. An ultrasonic microscope of this type is described in, e.g., Published Unexamined Japanese Patent Application No. 62-87854.
In this ultrasonic microscope, an ultrasonic pulse generated by a pulse circuit is radiated on a sample, an echo from the sample is received by a reception circuit, and the echo is converted into an electrical echo signal. A trigger signal is generated by the first echo signal having a value exceeding a threshold value when a predetermined period of time elapses from oscillation of the ultrasonic pulse. A gate signal is generated on the basis of this trigger signal. The gate signal is supplied to a gate circuit through which the echo signal is gated. The gate circuit is enabled at a timing and for a period of time determined by the gate signal, thereby extracting an echo component at a predetermined depth from the echo signal.
As described above, since the threshold value is set such that an echo component corresponding to an echo from the surface of the sample exceeds the threshold value for the first time upon oscillation of the ultrasonic pulse, only the echo component at the predetermined depth from the sample surface can be extracted. This echo component is converted into an image to obtain an observation image at the predetermined depth from the sample surface.
An actual time lag of several hundreds of nsec is present from the time when the echo signal from the surface of the sample exceeds the threshold value to the time when the gate circuit is enabled. Each echo from the boundary between the first and second layers inside the sample such as an FRP is reflected with a delay of about 100 nsec from the surface echo.
When the gate signal is generated by using the echo signal from the sample surface as a trigger signal as in the conventional ultrasonic microscope, no echo can be extracted from this boundary. This indicates that an observation image at the boundary between the first an second layers cannot be obtained.
In the conventional ultrasonic microscope described above, the echo from the sample surface cannot be gated, and an observation image on the sample surface cannot be obtained, either.
In addition, as shown in FIG. 5, when a target boundary 1 is not present at a predetermined depth from the surface of the sample, an image at the boundary 1 cannot be obtained even if an echo from the sample surface is used as a trigger signal.
In the internal structure shown in FIG. 5, it is preferable that an echo from a boundary 2 is used as a trigger signal, and a previous echo from the boundary 1 is gated using the above trigger signal. The conventional ultrasonic microscope cannot generate such a gate signal.
As shown in FIG. 6, if a boundary in a sample is stepped and condition L1=L2 is established, an echo from a boundary 1 for a portion A returns at the same timing as that for a portion B. Therefore, the resultant images at the portions A and B are the same, resulting in inconvenience.
When no echo returns from the surface of the sample or exceeds the threshold value, a gate signal is generated using internal reflection of an acoustic lens as a trigger signal. As a result, an accurate observation image cannot be obtained.